Patent Document 1 teaches the occurrence of abnormal combustion in an internal combustion engine by simultaneous introduction of engine lubricating oil and intake air into a combustion chamber in a low-speed high-load range. It is herein described that oil mist entrained in blow-by gas remains within an intake surge tank of the engine and flows together with intake air stream into the combustion chamber. In order to prevent the occurrence of abnormal combustion due to such oil component, there is disclosed a technique of estimating the amount of oil flowing into each cylinder and performing boost pressure control, air-fuel ratio enrichment, ignition timing retardation etc. according to the estimated oil amount. In particular, Patent Document 1 considers that the probability of occurrence of abnormal combustion due to oil component increases with cylinder bore wall temperature as in the case of usual knocking and thus proposes increasing the degree of boost pressure control or air-fuel ratio enrichment in response to increase in cylinder bore wall temperature.
There is however a possibility of intense abnormal combustion (one sort of pre-ignition phenomenon) accompanied by extraordinary noise, which is different from usual knocking, at the time when a supercharger-equipped internal combustion engine, notably of cylinder direct injection type, shifts into a supercharging region at a low rotation speed e.g. with the depression of an accelerator pedal by a driver during warm-up operation where engine temperature is low.
The above-mentioned abnormal combustion could occur even without the inflow of oil component from the intake system. The present inventor has found, as a result of research, that the above-mentioned abnormal combustion is a different type of abnormal combustion that occurs by a different mechanism from that described in Patent Document 1.
The following are the findings of the research made by the present inventor. In a state where a cylinder bore wall is relatively low in temperature, fuel (in general, gasoline) injected from a cylinder direct injection fuel injector adheres in the form of droplets onto a surface of the cylinder bore wall; whereas oil is present in the form of a very thin film on the surface of the cylinder bore for lubrication of a sliding surface of a piston. With upward movement of the piston, the oil and the fuel droplets gather in a crevice above an uppermost piston ring of the piston (i.e. a slot-like space having three sides surrounded by an outer circumferential surface of the piston, the surface of the cylinder bore wall and the piston ring) so that the oil is diluted with the fuel droplets. When the piston moves upward in this state during a compression stroke, the speed of the piston increases in the first half of the compression stroke but decreases from the midpoint of the compression stroke. The oil-fuel mixture is then released from the crevice into the combustion chamber under inertia force. As the air-fuel mixture inside the combustion chamber has become high in temperature and pressure by compression in the latter half of the compression stroke, the released oil component act as an ignition source to cause ignition of the air-fuel mixture before the normal ignition timing. As a result, there occurs abnormal combustion. In particular, the air-fuel mixture is high in temperature in the vicinity of a top dead center of the compression stroke in a supercharging region. Further, the actual time for the oil to be ignited as the ignition source is long at a low engine speed. The abnormal combustion is thus likely to occur in a supercharging region at a low engine speed. Although the oil is lower in volatility than the fuel, the ignitability of the oil, when vaporized, is higher than that of the fuel so that the oil readily acts as the ignition source by release into the combustion chamber.
Basically, the above-mentioned mechanism of abnormal combustion does not occur after warm-up operation of the internal combustion engine, i.e., in a state where the temperature of the cylinder bore wall is high. It is because, when the temperature of the cylinder bore wall is sufficiently high, the injected fuel rapidly evaporates upon collision with the surface of the cylinder bore wall and does not get mixed in liquid form with the oil in the crevice and because the oil solely accumulated in the crevice maintains its high viscosity without being diluted with the fuel and does not fly into the combustion chamber with the stroke movement of the piston. Accordingly, the above-mentioned abnormal combustion becomes a problem only when the engine is in an unwarmed state where the cylinder bore wall is low in temperature. The lower the temperature of the cylinder bore wall, the more likely it becomes that the above-mentioned abnormal combustion will occur.
The present invention has been made based on these new findings. It is an object of the present invention to prevent the occurrence of abnormal combustion in a cylinder direct injection type internal combustion engine with a supercharger in a low-speed supercharging region under low engine temperature conditions.